(U) Microwave repeaters in various forms have been used for many years. Repeaters are frequently used as electronic countermeasures to sophisticated radar systems.
In electronic warfare (EW) applications, EW repeaters receive an input signal from a distant enemy pulse or continuous wave radar system, modulate and amplify the received input signal, and then retransmit the modulated and amplified input signal, i.e., the output signal of the EW repeater. Modulation of the input signal includes phase, amplitude and frequency modulation. EW repeaters require both high gains and short time delays, e.g., a time corresponding to fraction of a pulse width, so that the processing logic of the enemy radar system is unable to distinguish between the retransmitted input signal of the EW repeater and the electromagnetic return signal from a target of interest.
Referring to FIG. 1, a conventional microwave repeater 1 comprises an antenna cluster 10, which includes a receiving antenna 12 coupled to a transmitting antenna 16 via both an amplifier section 14 and the ground plane. Amplifier section 14 includes a conventional traveling wave tube (TWT) or solid state amplifier 18 and a conventional modulator 20. The input signal from the distant enemy radar system received by antenna 12 is modulated in modulator 20, amplified in amplifier 18, and then retransmitted b antenna 16.
Microwave repeater 1 has a predetermined mutual coupling in the ground plane between antennas 12 and 16, i.e., the inverse of the isolation between the antennas. The gain of amplifier section 14 is maintained below the absolute value of the antenna isolation in order to prevent repeater 1 instability due to feedback from antenna 16 into antenna 12.
It is desirable for the gain of repeater 1 to be as large as possible so that the retransmitted input signal is interpreted as a large target by the enemy radar system. The limiting factor on the gain of repeater 1 is the isolation between antennas 12 and 16, not the gain achievable by amplifier section 14. Referring to FIGS. 1 and 2, conventional methods for improving the isolation between antennas 12 and 16 are illustrated. In FIG. 1, the primary isolation method is spatial separation between antennas 12 and 16, which provides spatial attenuation. In FIG. 2, isolation is achieved both by spatial separation and by a conventional ferrite absorber 22, which is located in the ground plane between antennas 12 and 16.
The basic problem with the conventional isolation methods shown in FIGS. 1 and 2 is providing a uniform impedance match between antenna 16 and absorber 22 over a broad frequency range. The conventional methods which have heretofore been used provide an ideal impedance match in only one frequency band, while providing large impedance mismatches in other frequency bands of the overall frequency range.
(U) Conventional interference cancellation methods using a main antenna and one or more auxiliary antennas, such as that disclosed in U. S. Pat. No. 4,893,350, require weighting circuitry for adjusting the amplitude of the undesired signal received at the auxiliary antenna as well as canceller circuitry for subtracting the adjusted undesired signal from the signal received at the main antenna. Since the signal received at the main antenna contains both the desired and the undesired signals, and since the undesired signals from both antennas are set equal to one another by the weighting circuitry, the canceller circuitry effectively removes the undesired signal. However, the circuitry required for weighting and cancelling, especially the circuitry for complex weighting of microwave signals, increases the time delay for coupling the desired signal from the main antenna to downstream amplifiers and processors.